temperature gauge fluctuates

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New Member
Nov 26, 2023
W204 220cdi om651
I have a 2014 C220 CDI. The temp normally stays the 90C mark.

However recently its been getting to that mark, and after about 40 minutes of driving it'll slowly drop to 80-85c mark, then slowly raise back to where it was. This happens on almost every journey now, sometimes multiple times depending how long the journey is. It can be at high speed or low speed, so its not dependant on engine load. No difference with ac/heater on/off. New merc thermostat installed and still the same. No leaks, proper coolant 325.00, 50/50. However, when it happened last time, I noticed that the fun was running with 10c air temp and coolant under 90c? Can someone tell me which sensor is responsible for turning on/off the cooling fan please? Is it one of those on the thermostat? cheers

Perfectly normal, especially in these colder winter temps.

The OM651 has a "mapped thermostat" and a electronically controlled water Pump. The temp will fluctuate for two reasons. Efficiency, and power demands. Mine always drops if I use full throttle for a time and also after a long run when coming off the motorway. Has been like this for 5 years, 40,000 miles, more noticeable in the winter.

The fan can run for other reasons, such as air con demands etc.

All sounds perfectly normal to me.

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